The real future of TV: how IPTV works

Australia’s TV broadcasting landscape may have changed forever in 2010 with the switch off of analogue broadcasts in Mildura Victoria but it was the launch of pay-TV rival Fetch TV and Foxtel’s service to broadcast content to Xbox 360s that breathed life into IPTV â€“ the real future of TV.

It’s easy to confuse them but when it’s done right IPTV is a world away from the click-and-watch paradigm that’s dominated by YouTube and its competitors. IPTV uses an IP network like the internet â€“ but in commercially available services not the internet so much as the internal network that links ISPs with their customers â€“ to carry a roster of standard or high-definition broadcasts.

While ISPs like TPG have run IPTV services for several years their lack of mainstream content has generally limited their appeal; the Fetch TV and Foxtel services however have built on growing awareness of internet-delivered catch-up TV and brought mainstream content into households via IPTV for the first time.

The set-top box you plug into your TV is just the end point in a long IP
Multicast chain that can distribute content to large numbers of users
without flooding ISPs’ networks.

All going well this should see subscriber numbers escalate rapidly in coming years particularly as IPTV is taken up by households that are unable to get full-fledged pay TV through other methods. Yet while essential for the commercial viability of IPTV services this growth also puts pressure on ISPs to engineer their networks to ensure they won’t get swamped by the raw bandwidth demand posed by delivering thousands of video streams at once.

After all if 5000 subscribers to Internode’s FetchTV service all decide to tune into an HD broadcast of ‘Battlestar Galactica’ that uses around 6Mbps of bandwidth Internode needs to deliver 30000Mbps or 30Gbps of data over its internal network â€“ right?

Wrong. These obvious issues of scale apply to internet video where heavy usage puts strain on ISPs’ own internet connections and drives some ISPs to cache data in on-network content distribution networks (CDNs). But when it comes to full commercial IPTV services service providers are pulling a 22-year-old IP protocol out of the closet dusting it off and using it to make sure your IPTV services are as smooth and robust as the ones you get over the air or over your pay-TV cable.

“Cable is a broadcast network and all the content is flooding out all the time” says Nick Turner senior content engineer with iiNet whose FetchTV-powered IPTV offering has picked up over 1500 subscribers since its launch in mid 2010. “The reason that has been sustainable is that the coax has enough bandwidth for all that content with a bit left over for data. With ADSL we’re quite limited although we have it to the point where we can run a STB to a customer with a 4Mbps or better connection.”

Blast from the past
Although video can reach users fine getting it there requires a bit of trickery. The secret sauce behind modern IPTV is ‘IP multicast’ capability which has been an intrinsic part of the official IP standards since 1989 but has been rarely used because most ISPs didn’t have much to do with it.

In the multicast world end nodes request to join or leave multicast ‘groups’ by sending signals according to the methods encapsulated in IGMP v3 (Internet Group Management Protocol aka IETF RFC 3376). Using IGMP endpoint devices communicate with their local router indicating their interest in joining specific multicast groups. The router then adds the device’s IP address to its internal routing tables. Whenever it receives data that’s been tagged as being part of that IGMP group the router runs through those routing tables and broadcasts the data to all of the end nodes that have requested to be part of the multicast group.

An IGMP message includes a number of specific bits of information including an 8-bit header indicating its type; an 8-bit timeout interval; a 16-bit checksum of the entire IGMP message; a 32-bit representation of the group address; 32 bits worth of control settings; and in some cases a list of 32-bit IP addresses specifying a number of content sources.

To make sure devices are still online and still want to receive multicast streams routers query all devices in their lists to update their membership status at regular intervals. In this way the routers keep a subscription list as it were for each different video stream they’re delivering.

This suits IPTV perfectly. FetchTV for example has 39 regular channels to which subscribers can tune in and other channels offering pay-per-view movies. Assuming each of these channels is being watched by someone somewhere an ISP carrying the FetchTV service can budget for somewhere between 78Mbps (assuming all are delivered in 2Mbps SD resolution) or 252Mbps (assuming all are delivered in 6Mbps HD resolution) in total backhaul consumption.

This data traverses the ISP’s internal network from wherever its broadcast point is located to key points of presence in capital cities and then on to customers’ local ADSL2+ exchanges. IGMP routing tables maintained at the POPs steer the video streams to each exchange that has nodes requesting them while routing capabilities built into DSLAMs â€“ the devices ISPs install in Telstra exchanges to provide ADSL2+ services â€“ keeps track of which subscribers to that exchange are watching which programs.

iiNet says Turner is currently pushing approximately 180Mbps of multicast IPTV data around its network at any given time with each exchange distributing around 20Mbps of data at once. Throughout its entire route data is prioritised over general internet traffic to ensure there is no jitter at the customer’s end.

“Based on the multicast model we knew we would be able to run out to our exchanges so long as we didn’t have every single customer watching every channel at once” he says. “We have dimensioned around 300Mbps overall dedicated to multicast so there’s never any chance of the core of the network to be interrupted. It’s running as rock-solid as we expected.”

The final step is the transmission of the content from the exchange to the set-top box in the consumer’s home. IPTV may be deliverable to computers but the need to manage access to subscription content means you’re unlikely to see FetchTV or any other IPTV service delivered straight to computers in the near future; doing so would require a compatible digital rights management (DRM)-capable Flash video viewer be used at every single web site which is far outside the realm of possibility.

“When we’ve got the set-top box we’ve got control of the player and it gives us complete control of the network end to end” says Turner.

Smarter networks
Because of the sheer number of endpoints on the general internet routers would trip over their proverbial shoelaces long before they could begin to deliver something even vaguely approximating multicast on the general internet. This is a major differentiator between multicast-based services and those of catch-up TV providers Foxtel’s Xbox 360 service and even the ‘freezone’ services that many ISPs use to offer unmetered access to specific types of content such as iTunes downloads or ABC iView programs.

These services follow the conventional ‘unicast’ model in which users click to receive certain data and the video is streamed on a one-to-one basis; freezones work by storing content in a specific IP range within the ISP’s network and exempting data requests to that range from bandwidth metering.

Unicast services scale linearly putting great pressure on ISPs’ core networks and potentially causing the congestion that every internet user has experienced at one time or another. Alleviating such congestion either requires: ISPs to lease access to more fibre backhaul to increase bandwidth; or intelligent caching techniques to push popular content to STBs during periods of low network usage (these techniques are also in use by FetchTV and other providers); or the creation of a smarter network that can extend multicasting into the general domain.

The evolving national broadband network (NBN) looks set to accomplish the latter: its design will incorporate multicast capabilities at every point meaning that ISPs leasing access services from administrative body NBN Co will be able to count on high-bandwidth pipes for delivering multicast video to any household in Australia at once. With the IP Multicast control mechanism already well-established and households soon to get more than adequate bandwidth we’ll all be able to watch high-quality IPTV from all kinds of sources.